I'd assume since he's indicating a "synth" problem & posting in the hardware forum, that he's having a hardware synth issue... Whrn he posts in the Cubase forums, he may be urged to post in the hardware forum.
If suspecting it's a keyboard/synth I would disconnect my keyboard from my Cubase DAW, and check it out with headphones from it's jack and connect the line output with an amp or Y splitter with headphones.
But then again, the midi issue sounds like a midi message assignment thing, which I've had before with my hardware synth & Cubase...in my case was partly that there was a scramble due to something changed beyond my control & and/or user error. At any event it was an easy fix once I simply took a look at my Cubase & keyboards settings, got on the right track/midi channels.... [hint]
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